Influence of gestational age and fetal heart rate on the fetal mechanical PR interval

The fetal mechanical PR interval obtained via pulsed Doppler has previously been demonstrated to correlate with electrocardiographic PR interval measured in the neonate. We sought to further analyze the influence of fetal heart rate and gestational age upon the fetal mechanical PR interval.

We searched our database for mechanical PR intervals, which were obtained during fetal echocardiography performed in our antenatal diagnostic unit. We included fetuses with a normal cardiac structural survey. The mechanical PR interval is measured from the A wave of the mitral valve to the beginning of ventricular systole corresponding to the opening of the aortic valve. Linear regression curves were generated to examine the correlation of mechanical PR interval with gestational age and fetal heart rate. Analysis of variance was used to compare the mean variation across three gestational age groups: 17-21.9 weeks (n = 24), 22-25.9 weeks (n = 52) and 26-38 weeks (n = 20).

Mechanical PR intervals were measured in 96 fetuses with normal fetal echocardiography. The mechanical PR interval was 123.9 +/- 10.3 ms (mean +/- SD), with a range of 90-150 ms. Linear regression curves correlating mechanical PR interval with fetal heart rate and gestational age demonstrated a flat slope with R2 = 0.016, p = 0.22 and R2 = 0.0004, p = 0.85, respectively. The mechanical PR interval measured over the three gestational ages was as follows (mean +/- SD): 122.3 +/- 10.5 ms for 17-21.9 weeks; 125.0 +/- 9.6 ms for 22-25.9 weeks; and 123.1 +/- 11.9 ms for 26-38 weeks. Analysis of variance revealed no difference among the mechanical PR interval means measured over the three gestational age groups (p = 0.53).

Fetal mechanical PR interval ranges from 90 to 150 ms in fetuses with sonographically normal fetal cardiac structure and rate. The mechanical PR interval appears to be independent of gestational age and fetal heart rate.